This invention originated from research performed on the feasibility of using ANR technology to improve the performance of audiology testing. Hearing tests are conducted using "pure tone audiometers" that are designed to deliver a single frequency test-tone to the test subject at varying level. The proctor varies the sound pressure level of the tone and interrogates the subject about the lowest level that is audible. That level is the hearing threshold level of the test subject for that frequency. It is intuitive that the background noise in the test chamber can interfere with this threshold measurement. When the background noise levels are higher than the test tone level (at the ear) the test tone can be "masked" and it will appear to the user that his/her threshold is higher than it would be in a quieter environment. This masking effect is relatively narrowband in nature, due to the physiology of the ear. Therefore, it is not required to suppress ALL the background noise in order to alleviate the tonal masking. It is required that the frequencies of the background noise that are "nearby" the test tone be suppressed. Therefore, it became clear that it is not only desirable to place the frequency of maximum ANR suppression at the test tone frequency, it is actually essential that the controller provide its maximum suppression in the frequency bandwidth(s) surrounding the test tone(s). Therefore, a switching controller was built and tested. It works as expected. During this project, it became apparent that all personal ANR systems could benefit from this type of ANR architecture.
In audiometry testing, it is desirable to occlude any ambient noise in the testenvironment in order to accurately identify the subject's minimum hearing threshold. Pure tone audiometry testing uses a unique set of single tones, standard for all pure tone audiometers, to identify this minimum threshold. As the tone varies, so must the controller design. Previously, the background noise masking has been reduced (or nearly eliminated) by using sound proof booths (called test booths) or by using a product known as ear inserts, or by a passive earcup (audiocup) installed over the hearing test equipment. The ANR technology performs as well, perhaps better, than some of these products. There are no ANR audiometers in the market place.The bode integral theorem limits the amount of control that can be realized across a wide range of frequencies for a single, fixed-gain controller (i.e. the classic ANR headset). This means that it would be beneficial to circumnavigate this problem by providing a master system with the ability to "call up" different feedback controllers that do not try to extend performance over a very broad bandwidth.
There are two distinct aspects to this invention. One is that no existing personal ANR systems rely on a switching controller. The other is that prior to this invention audiometers have never used ANR, The background prior art does not show any use of a personal ANR system (i.e. ANR headset, ANR communications headset, silent seat, etc.) that utilizes a series of controllers that can be switched by the user or some automation algorithm/hardware. The instant approach is clearly desirable if one does not have accurate knowledge of the acoustic noise that must be suppressed for the user. For example, the BOSE headset uses a fixed-gain controller with the maximum noise suppression occurring at approximately 200 Hz, and tapering off to no suppression with decreasing and increasing frequency. If the disturbance noise did not contain frequencies between 100 Hz and 300 Hz the BOSE controller wouldn't be very useful in suppressing noise. This switching controller invention relies on multiple fixed-gain controllers, designed as an entity called a switching controller. The user, or a method, can then switch to the particular fixed-gain controller design that performs best for the noise field impinging upon the user. This should lead to the best reduction of background noise, using either electronic measurements or psychoacoustic perception metrics such as loudness.
Having described the invention in general terms, the objects of the invention are related below.